Tangent from Geometry node ignores seams from uv map
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I have just created a node group that can apply a normal map to a shader [1] and i noticed that with r50470 [2] Cycles is now able to create correct tangents for transformed objects. But it still ignores seams or just interpolates wrongly. If i look at the output of this node then i see that there is some kind of interpolation from vertex to vertex happening (like with vertex colors), but if i rip the UV apart and rotate single faces (tris) then i would expect a hard border from this face to the neighbor faces. That means: Interpolation should only happen inside a face or uv island, but a vertex should have different tangents depending on to which face it belongs and the face is oriented inside the uv map.

I appended an example file in which the top left faces should be solid red and the bottom right face should be solid green (no visible interpolation at all), but it has a lot of orange, which shouldn't be there.

[1] http://blenderartists.org/forum/showthread.php?216113-Brecht-s-easter-egg-surprise-Modernizing-shading-and-rendering&p=2222490&viewfull=1#post2222490
[2] http://lists.blender.org/pipermail/bf-blender-cvs/2012-October/050470.html



The behavior is intentional, and it's the same as blender internal. If you want seams, you need to use an edgesplit modifier, same as when you want seams for normals. The tangent here is intended for anisotropic shading, not for normal mapping. When we add normal mapping support a tangent suitable for that would be available separately.